Lithium-ion cells have at least one positive electrode and one negative electrode (cathode and anode, respectively), which can reversibly intercalate or deintercalate lithium ions (Li+). The terms lithium-ion cell, lithium-ion polymer cell, lithium-ion cell, battery, rechargeable battery and system are used largely synonymously.
Lithium-ion rechargeable batteries have to meet demanding requirements, in particular in the automotive sector, with respect to the gravimetric energy density (given in Wh/kg), in order for example to achieve the greatest possible ranges for vehicles driven by electric motors. The nominal capacity of a lithium-ion cell is determined by what are known as the active materials, for example lithiated transition metal oxides such as lithium-nickel oxide (LiNiO2) in the positive electrode and graphites or carbons in the negative electrode. However, in every lithium-ion cell there are also what are known as passive materials or “dead materials”, which consequently have an influence on the energy density of the lithium-ion cell. These include, for example, electrically conductive material, electrode binders, separators, outgoing conductor foils, collectors, terminals and the housing of the cell or of the rechargeable battery itself.
Outgoing conductor foils serve the purpose of electrically contacting and bonding the positive electrode and the negative electrode, the contact with the terminals being made by means of the collectors.
The construction of a lithium-ion cell with outgoing conductors, collectors and the external terminals is known.
In lithium-ion cells, aluminum-based outgoing conductor foils, collectors and terminals are usually used for the positive electrode side. On the negative electrode side, copper, nickel or nickel-plated copper is used, since otherwise, if aluminum were used on the negative side, there would be an alloying of lithium and aluminum on account of the potential that is present.
The use of copper or nickel as a passive material in a lithium-ion cell results in higher production costs. Moreover, this constitutes the use of passive materials, which have a higher crystallographic density in comparison with aluminum. Thus, aluminum has a density of 2.7 g/cm3 and copper and nickel have a density of 8.9 g/cm3. Accordingly, the use of copper or nickel as materials in lithium-ion rechargeable batteries has an adverse effect on the gravimetric energy density.